The cardiac repolarization period of the cardiac cycle, primarily consisting of the T-wave, is of interest for a variety of uses, including the analysis of cardiac function. For instance, repolarization abnormalities can be associated with dangerous arrhythmias, which are desirably detected for use in assessing cardiac function, ongoing health monitoring and/or treating cardiac pathologies. The QT interval (the time between the start of a Q-wave and the end of a T-wave) is frequently measured as an indicator of repolarization time with longer-than-normal or shorter-than-normal QT interval associated with possible risk of life-threatening arrhythmias. Evaluation of QT interval as an indicator of risk of life-threatening arrhythmias can involve measurement of average QT interval, QT interval dynamics, or both. Regulatory agencies can require that QT interval be measured in both animal models and human subjects during the course of developing new drugs as a means of assessing potential for drug-induced arrhythmias. QT interval measurements are also used to guide therapies in clinical care. Beyond measurement of QT interval, cardiac repolarization can be evaluated for clinical care and research using other methodologies including T-wave alternans, T-wave complexity, T-wave variability, and T-wave morphology changes.
Accurate measurement of QT interval has been challenging as a result of difficulties in accurately and consistently identifying T-wave offset due to its flat pattern, especially in the presence of noise. The accuracy of results produced by current methods is compromised, however, by noise in the ECG and by difficulty in accurately identifying T-wave offset. Further, approaches to identifying T-wave offset have suffered from an inability to accurately determine whether a particular T-wave offset is accurate, or whether the result may have been compromised due to the presence of noise, certain arrhythmias or difficult repolarization wave morphology. These and related matters have presented challenges to the measurement of QT interval, assessment of QT interval dynamics, and isolation of the cardiac repolarization signal of an ECG.